The present invention is directed to a method for controlling color for a printing press using at least one ink-feeding device. In addition, the present invention is directed to a measuring device having a detector for measuring spectral reflectance values on at least one printed surface element on a print substrate, an associated control unit including a processor unit and a memory unit; the present invention also relates to a printing system having at least one printing press, which includes at least one print unit, an ink-feeding device, and a machine-control unit.
Controlling the ink application in a printing press is an important way to influence the printing result. To analyze the printing result, from which operational principles for controlling the ink application are derived, color-control fields, whose chromatic values are determined by visual assessment or by taking measurements on the surface elements, are often printed in the same print job on surfaces elements of the print substrate (paper, cardboard, organic polymeric foil or the like). One way to accomplish this is to determine the spectral reflectance β(λ) of the surface elements. In the notation employed here, β(λ) signifies that the reflectance β is a function of the wavelength λ. From the spectral reflectance, colormetric values or density values can be calculated. For this, standard specifications have been issued in Germany. Colormetric values can be defined on the basis of the German Industrial Standard DIN 16 536, and density values on the basis of the German Industrial Standard DIN 5033.
From European Patent No. 0 228 347 B2, a method for controlling the ink application of a printing press, as well as a measuring device and a printing system are known. To control the ink application, surface elements are measured colormetrically on a print substrate printed on by a printing press, and the color coordinates obtained are processed, in combination with setpoint values, into control data for ink-feeding devices of the printing press. The light reflected off of the surface elements is spectrally dispersed and measured in a spectrometer. The measuring data obtained at discrete points of reference of different wavelengths are fed to a computer. The control is carried out on the basis of spectral color measurement and colorimetry, in that, optionally in a conversion operation, chromatic values in a color-coordinate system are determined from the reflectance values. Actual values are compared to setpoint values, and deviations in the spectral reflectance or in the chromatic values are reduced by the color control.
Spectral reflectance can be measured either in an unpolarized or polarized operation. In other words, polarized, in particular linearly polarized light can be optionally used for illumination purposes, and a detector can be equipped with polarization optics or with a polarizer to measure polarized, reflected light. Typically, the light is measured using linear polarization rotated by 90 degrees; this is a component of the depolarized, reflected light. However, because of technical limitations, it is not always possible to equip detectors with polarization optics. Furthermore, polarization optics or polarization spectrometers constitute a considerable cost factor. Polarized spectral reflectance is an example of a variable whose measurement entails substantial outlay.
However, knowledge of the polarized spectral reflectance is vital, since it is independent of the drying state of the print substrate. The spectral reflectance must often be measured during or immediately after the printing operation, which means, particularly in present-day offset printing, that the print substrate has a specific moisture content. The moisture content decreases too slowly for it to be useful for analysis of the printing result. A color control on the basis of polarized spectral reflectance values or on the basis of chromatic values determined on the basis of polarized spectral reflectance values, implies setpoint values which are independent of the drying state of the print substrate and, thus, time-invariant following the printing operation. Thus, if the setting of a printing press to desired setpoint values is tracked for a print production, then during or immediately following the printing operation, actual values of polarized spectral reflectance or chromatic values derived therefrom can be compared to the setpoint values and ink-feeding devices can be controlled until the ink supply in the printing press is such that the deviation between actual values and setpoint values is imperceptible to the point of being sufficiently precise. It is often the color difference ΔE in the underlying color space that is regarded as a measure of sufficient precision. When ΔE<1±0.5, the color difference is below the threshold of perception or visibility. Even for a length of time following the printing operation, this result does not fundamentally change, since the color control is based on time-invariant variables.
Creating a physical model to describe light-scattering processes in print substrates is extremely difficult, due to the optical properties of customary print substrates. This can be inferred, for example, from the article by G. Fischer, J. Rodriguez-Giles and K. R. Scheuter in “Die Farbe” [Color] 30 (1982), pp. 199 through 220. To mention just a few examples of how light is affected, on the one hand, the light that is incident to the print substrate is not only scattered directly at the surface, but can also be scattered, in part, inside the surface layer of the print substrate, and, on the other hand, the light is not only scattered on the way into the print substrate, but can also be scattered on its way out of it again. Thus, the light paths through the surface layer of a print substrate are very complicated, and the reflectance behavior resulting therefrom can only be calculated in simple cases and not globally. For that reason, there seem to be insurmountable limits placed on a calculation of the polarized spectral reflectance, in particular on a universal type of calculation for various print substrates.